1. Field of the Invention
The present invention relates to a lens system in facsimiles and copying machines, and more specifically, to a Gaussian lens comprising four groups including six lenses.
2. Description of the Prior Art
Recently, a high-density solid image forming element has been developed for putting it to practical use. For example, suppose that a solid image forming element of 7 .mu.m is used, a resolving power of 71.4 lines/mm is required at the image surface. Thus, it is quite difficult to maintain a level of the same high performance from the center to the marginal portion.
If, for practical use, an attempt is made to decrease a synthesized thickness of the second and third lens groups of the Gaussian lens comprising four groups including six lenses, there occurs two problems. One problem is that the Petzval's sum increases, and the other is that a thickness of a lens side of a positive lens system decreases to fail to secure brightness and an angle of view, and so, if an absolute value of a radius of curvature of a cemented surface is intended to be reduced, chromatic aberration is turned negative.
Prior art lenses comprising four groups including eight lenses are disclosed, for example, in U.K. Pat. Nos. 658,349 and 678,838. However, these patents have a problem that the aberration amount is large, failing to be a high performance lens.